Marine propulsion arrangements as described above come chiefly in two types, i.e. purely outboard motors in which the engine is rigidly connected to the upper end of the propeller drive shaft housing, and so-called stern drive installations in which a stern drive unit outboard of the transom is connected to a motor located inboard of the transom. In the first-mentioned type, the carrier is usually in the form of a clamp arrangement which clamps over an edge of the transom. The motor and drive shaft housing are carried by the fork-shaped support bracket. In the second type, the carrier is normally in the form of an attachment plate or frame which is mounted in an opening in the transom and to which the fork-shaped support bracket is pivotally journalled to impart pivotal movement to the drive shaft housing relative to the transom.
Particularly in stern drive installations, pivotal displacement of the drive shaft housing for steering purposes is achieved by rotation of a steering spindle to which a steering arm is affixed. The spindle is nonrotatably attached to the drive shaft housing and journalled in the support bracket. By means of a push-pull cable connected directly to the steering arm, or a hydraulic servo-unit, rotation of the steering wheel of the boat effects displacement of the steering arm, with consequential rotation of the steering spindle and displacement of the drive shaft housing relative to the transom. Due to the relatively short steering arm, in this type of construction high steering forces are required, particularly at high engine power levels, thereby leading to high stresses. The system must therefore be overdimensioned to prevent deflections and wear from becoming unacceptable.
In a known arrangement in which longer steering arms are employed in order to reduce the steering forces, use is made of a pair of hydraulic cylinders which are connected to the boat transom and regions of the propeller drive shaft housing at a distance from the steering spindle. Such an arrangement does however suffer from certain problems. The steering arrangement is not fully integrated in the propeller drive unit since one end of each hydraulic cylinder must be attached to the transom. The installation is therefore dependent on the form of the transom and the location of other possible components mounted thereon. The choice of attachment points on the transom is restricted since these should lie along the pivot axis for the trim displacement of the drive unit.